High Strength Carbon Fiber Composite Wafers For Microfabrication

ABSTRACT

A method of making a high strength carbon fiber composite (CFC) wafer with low surface roughness comprising at least one sheet of CFC including carbon fibers embedded in a matrix. A stack of at least one sheet of CFC is provided with the stack having a first surface and a second surface. The stack is pressed between first and second pressure plates with a porous breather layer disposed between the first surface of the stack and the first pressure plate. The stack is cured by heating the stack to a temperature of at least 50° C.

CLAIM OF PRIORITY

This is a divisional of U.S. patent application Ser. No. 13/667,273,filed Nov. 2, 2012; which claims priority to U.S. Provisional PatentApplication Ser. No. 61/689,392, filed on Jun. 6, 2012; and which is acontinuation-in-part of U.S. patent application Ser. No. 13/453,066,filed on Apr. 23, 2012, which claims priority to U.S. Provisional PatentApplication Nos. 61/486,547, filed on May 16, 2011; 61/495,616, filed onJun. 10, 2011; and 61/511,793, filed on Jul. 26, 2011; all of which arehereby incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present application is related generally to high strengthmicrostructures, such as for example x-ray window support structures.

BACKGROUND

Carbon fiber composite (CFC) wafers can be used in applications wherehigh strength is desired. Barriers to the development of carbon fiberbased structures, especially structures with micrometer-sized features,include difficulties in machining or patterning, and high surfaceroughness of cured composites. A root mean square surface roughness Rqof typical CFC wafers can be greater than 1 micrometer. Root mean squaresurface roughness Rq can be defined by the following equation:R_(q)=√{square root over (Σz_(i) ²)}. In this equation, z represents aheight of the surface at different measured locations i.

SUMMARY

It has been recognized that it would be advantageous to have a carbonfiber composite wafer having high strength and low surface roughness.

In one embodiment, the present invention is directed to a method ofmaking a carbon fiber composite (CFC) wafer that satisfies the needs forhigh strength and low surface roughness. The method comprises pressing astack of at least one sheet of CFC between pressure plates with a porousbreather layer disposed between at least one side of the stack and atleast one of the pressure plates; then heating the stack to atemperature of at least 50° C. to cure the stack into a CFC wafer.

In another embodiment, the present invention is directed to a carbonfiber composite (CFC) wafer that satisfies the needs for high strengthand low surface roughness. The CFC wafer comprises at least one sheet ofCFC including carbon fibers embedded in a matrix. The wafer can have athickness of between 10-500 micrometers. The wafer can have a root meansquare surface roughness Rq, on at least one side, of less than 300 nmin an area of 100 micrometers by 100 micrometers and less than 500 nmalong a line of 2 millimeter length. The wafer can have a yield strengthat fracture of greater than 0.5 gigapascals, wherein yield strength isdefined as the force, in a direction parallel with a plane of the wafer,per unit area, to cause the wafer to fracture. The wafer can have astrain at fracture of more than 0.01, wherein strain is defined as thechange in length caused by a force in a direction parallel with a planeof the wafer divided by original length.

In another embodiment, the present invention is directed to an x-raywindow including a high strength support structure. The x-ray window cancomprise a support frame defining a perimeter and an aperture with aplurality of ribs extending across the aperture of the support frame andcarried by the support frame. Openings exist between the plurality ofribs. The support frame and the plurality of ribs comprise a supportstructure. A film can be disposed over, can be carried by, and can spanthe plurality of ribs and can be disposed over and can span theopenings. The film can be configured to pass x-ray radiationtherethrough. The support structure can comprise a carbon fibercomposite material (CFC). The CFC material can comprise carbon fibersembedded in a matrix. A thickness of the support structure can bebetween 10-500 micrometers. A root mean square surface roughness Rq ofthe support structure on a side facing the film can be less than 500 nmalong a line of 2 millimeter length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of a carbon fiber composite wafer, inaccordance with an embodiment of the present invention;

FIGS. 2-3 are schematic cross-sectional side views of a carbon fibercomposite wafer, in accordance with an embodiment of the presentinvention;

FIG. 4 is a schematic cross-sectional side view of portion of a carbonfiber composite wafer, showing measurement of root mean square surfaceroughness Rq, in accordance with an embodiment of the present invention;

FIG. 5 is a side view of a carbon fiber, in accordance with anembodiment of the present invention;

FIG. 6 is a schematic cross-sectional side view of wafer includingmultiple carbon fiber composite sheets abutting a polyimide sheet, inaccordance with an embodiment of the present invention;

FIG. 7 illustrates a first curing process for manufacture of a carbonfiber composite wafer, in accordance with a method of the presentinvention;

FIG. 8 illustrates use of o-rings and a vacuum during the first curingprocess for manufacture of a carbon fiber composite wafer, in accordancewith a method of the present invention;

FIG. 9 illustrates a second curing process for manufacture of a carbonfiber composite wafer, in accordance with a method of the presentinvention;

FIG. 10 is a schematic top view of an x-ray window support structure, inaccordance with an embodiment of the present invention;

FIG. 11 is a schematic cross-sectional side view of an x-ray window, inaccordance with an embodiment of the present invention;

FIG. 12 is a schematic cross-sectional side view of an x-ray detector,including an x-ray window, in accordance with an embodiment of thepresent invention.

DEFINITIONS

As used herein, the term “carbon fiber” or “carbon fibers” means solid,substantially cylindrically shaped structures having a mass fraction ofat least 85% carbon, a length of at least 5 micrometers and a diameterof at least 1 micrometer.

As used herein, the term “directionally aligned,” in referring toalignment of carbon fibers with support structure members (such as ribsfor example), means that the carbon fibers are substantially alignedwith a longitudinal axis of the support structure members and does notrequire the carbon fibers to be exactly aligned with a longitudinal axisof the support structure members.

As used herein, the term “porous” means readily permeable to gas.

DETAILED DESCRIPTION

Illustrated in FIGS. 1-3 are carbon fiber composite (CFC) wafers 10 and20 comprising at least one CFC sheet 21 including carbon fibers 12embedded in a matrix 11. The matrix can comprise a material thatprovides sufficient strength and is compatible with the use of thewafer. For example, if the wafer will be used to fabricate an x-raywindow support structure, considerations for matrix material may includea low atomic number elements and low outgassing. The matrix can comprisea material selected from the group consisting of polyimide,bismaleimide, epoxy, or combinations thereof. The matrix can comprise amaterial selected from the group consisting of amorphous carbon,hydrogenated amorphous carbon, nanocrystalline carbon, microcrystallinecarbon, hydrogenated nanocrystalline carbon, hydrogenatedmicrocrystalline carbon, or combinations thereof. The matrix cancomprise a ceramic material selected from the group consisting ofsilicon nitride, boron nitride, boron carbide, aluminum nitride, orcombinations thereof.

The carbon fibers 12 can be directionally aligned in a single directionAl, directionally aligned in multiple directions, or disposed in randomdirections in the matrix. Three CFC sheets 21 a-c are shown in FIGS.2-3. There may be more or less CFC sheets 21 than 3, depending on thedesired application. The wafer 20 can have a thickness Th_(w) of between10-500 micrometers in one aspect, between 20 and 350 micrometers inanother aspect, less than or equal to 20 micrometers in another aspect,or greater than or equal to 350 micrometers in another aspect.

CFC wafers per the present invention can have high yield strength. Ayield strength at fracture can be greater than 0.1 gigapascals (GPa) inone aspect, greater than 0.5 GPa in another aspect, greater than 2 GPain another aspect, between 2 GPa and 3.6 GPa in another aspect, orbetween 0.5 GPa and 6 GPa in another aspect. Yield strength can bedefined as a force F in a direction parallel with a plane 33 or 34 of aside 32 a or 32 b of the wafer, per unit area, to cause the wafer tofracture. If fibers are directionally aligned, the force F can bealigned parallel with the fibers.

CFC wafers per the present invention can have high strain. A strain atfracture can be greater than 0.01 in one aspect, greater than 0.03 inanother aspect, greater than 0.05 in another aspect, or between 0.01 and0.080 in another aspect. Strain can be defined as the change in length Lcaused by a force F in a direction parallel with a plane 33 or 34 of thewafer divided by original length L. If fibers are directionally aligned,the force F can be aligned parallel with the fibers.

The wafer can have two faces or sides 32 a-b and an edge 31. The sides32 a-b can have a substantially larger surface area than the edge 31.The sides 32 a-b can be substantially parallel with each other. One side32 a can be disposed along, or parallel with, a single plane 33; and theother side 32 b can be disposed along, or parallel with, a differentsingle plane 34.

At least one side 32 a and/or 32 b of the wafer can be smooth, i.e. canhave a low surface roughness. A low surface roughness can be beneficialfor improving adhesion to other materials, such as to an x-ray windowfilm for example. One measurement of surface roughness is root meansquare surface roughness Rq calculated by the equation R_(q)=√{squareroot over (Σz_(i) ²)}. The measurement z_(i) can be made along a surfaceof the wafer by an atomic force microscope. The measurement of z_(i) ona portion of the wafer 40 is shown in FIG. 4. A distance from a plane43, substantially parallel with the wafer, or substantially parallelwith the sides 32 a and 32 b of the wafer, can differ by small amounts.These small variations can be recorded, squared, summed, then a squareroot may be taken of this sum to calculate root mean square surfaceroughness Rq. A low Rq number can indicate a low surface roughness. Theroot mean square surface roughness Rq of one or both sides of the wafersof the present invention can be less than 300 nm in one aspect, orbetween 30 nm and 300 nm in another aspect, in an area of 100micrometers by 100 micrometers. The root mean square surface roughnessRq of one or both sides of the wafers of the present invention can beless than 500 nm in one aspect, or between 50 nm and 500 nm in anotheraspect, along a line of 2 millimeter length. The root mean squaresurface roughness Rq of one or both sides of the wafers of the presentinvention can be less than 200 nanometers in one aspect, or between 20nm and 200 nm in another aspect, in an area of 100 micrometers by 100micrometers.

Shown in FIG. 5 is a side view of a carbon fiber 12, in accordance withan embodiment of the present invention. At least 50% of the carbonfibers 12 in a wafer can have a diameter D of between 2 and 10micrometers in one aspect. At least 90% of the carbon fibers 12 in awafer can have a diameter D of between 2 and 10 micrometers in anotheraspect. Substantially all of the carbon fibers 12 in a wafer can have adiameter D of between 2 and 10 micrometers in another aspect.

As shown on wafer 60 in FIG. 6, a polyimide sheet 61 can be curedtogether with and can abut the sheet(s) 21 of carbon fiber composite.The polyimide sheet can have a thickness Th_(p), after curing, ofbetween 0.1-100 micrometers.

Also shown on wafer 60 in FIG. 6 are carbon fiber composite sheetthicknesses Th_(a-c). Each carbon fiber composite sheet 21 a-c in thestack can have a thickness Th_(a-c) of between 20 to 350 micrometers (20μm<Th_(a)<350 μm, 20 μm<Th_(b)<350 μm, and 20 μm<Th_(c)<350 μm) in oneaspect, less than or equal to 20 micrometers in another aspect, orgreater than or equal to 350 micrometers in another aspect. There may bemore or less than the three carbon fiber composite sheets 21 a-c. Thesethicknesses are sheet 21 thicknesses after curing.

FIG. 7 illustrates a first curing process 70 for manufacture of a carbonfiber composite wafer, in accordance with a method of the presentinvention. The method can comprise providing a stack 71 of at least onesheet of CFC 21 a-c, the stack having a first surface 32 a and a secondsurface 32 b; pressing P the stack between a first pressure plate 76 aand a second pressure plate 76 b with a porous breather layer 72disposed between the first surface 32 a of the stack and the firstpressure plate 76 a; and curing by heating the stack 71 to a temperatureof at least 50° C. (defining a first curing process). The amount ofpressure to be used can depend on the matrix of the carbon composite.Pressure in the range of 50-200 psi has been successfully used. Pressuremay be in the range of 25-500 psi.

A solid, polished layer 73 can be disposed between the second surface 32b of the stack 71 and the second pressure plate 76 b during the firstcuring process. The polished layer 73 can help create a very smoothsurface on the second surface 32 b of the stack 71. The polished layer73 can be a highly polished sheet of stainless steel, a silicon wafer,or a glass plate. A fluorine release layer can be used to avoid thestack sticking 71 to the polished layer 73. For example, a fluorinatedalkane monolayer can be deposited on silicon wafers to facilitaterelease by placing in a vacuum desiccator overnight with 5 mL ofTrichloro(1H,1H,2H,2H-perfluorooctyl)silane in a glass vial. Thepolished layer 73 can have a root mean square surface roughness Rq ofless than 300 nm in an area of 100 micrometers by 100 micrometers, on aside facing the stack. Thus, it is not necessary for the polished layer73 to have a polished surface on both sides.

A polyimide sheet 61 can be cured together with and can abut the CFCsheet(s) 21. The polyimide sheet 61 can be disposed between the secondsurface 32 b of the stack 71 and the second pressure plate 76 b. Thepolyimide sheet 61 can be disposed between the second surface 32 b ofthe stack 71 and the polished layer 73 (if a polished layer is used).Alternatively, a polyimide sheet 61 can be disposed on both surfaces 32a and 32 b of the stack 71. The polyimide sheet(s) 61 can be useful forimproving the surface of the final wafer and/or for improving adhesionof the stack 71 to other materials.

The porous layer 72 can allow gas, emitted by the stack, to escape fromthe press. A multi-layer porous breather layer 72 can be used. Forexample, the porous breather layer 72 can comprise a porous polymerlayer 72 b facing the stack 71 and a nylon mesh 72 a facing the firstpressure plate 76 a. A vacuum can aid in removal of the gas. A vacuumpump 75 can be attached by tubing 74 to the press and can draw a vacuum,such as less than 50 torr, between the pressure plates. The vacuum canbe maintained through substantially all of the curing process, orthrough only part of the curing process, such as at least 50% of thecuring process.

Shown in FIG. 8 are more details of the press and vacuum. The layers(stack of CFC, porous breather layer 72, optional polished layer 73, andoptional polyimide layer 61) 81 can be in a central portion of thepressure plates 76 a-b. An o-ring 82 can surround the layers 81. Theo-ring 82 can be disposed at least partly in a channel 83 of at leastone of the pressure plates 76 a and/or 76 b. The vacuum tube 74 canextend into the central portion of the press, between the layers 81 andthe o-ring 82.

FIG. 9 illustrates a second curing process 90 for manufacture of acarbon fiber composite wafer in accordance with a method of the presentinvention. After completion of the first curing process 70, pressure Pcan be released from the stack and the porous layer 72 can be removedfrom the stack. A polished layer 73 a and 73 b can be disposed on eachside of the stack. Note that if there was a polyimide sheet 61 in thefirst curing process, this polyimide sheet 61 can remain for the secondcuring process 90. The polished layers 73 a and 73 b can have a rootmean square surface roughness Rq of less than 300 nm in an area of 100micrometers by 100 micrometers, on a side facing the stack. The stack 71(and optional polyimide sheet 61 if one is used) can be pressed betweenthe polished layers by the first and second pressure plates 76 a-b. Thestack 71 (and optional polyimide layer 61) can be cured by heating thestack to a temperature of at least 50° C.

A benefit of use of the second curing process 90 is that the gas can beremoved during the first curing process 70, then polished layers 73 aand 73 b can be disposed on both sides 32 a and 32 b of the stack 71,with the result that both sides of the wafer can be highly polished.Thus, both sides of the wafer can have a root mean square surfaceroughness Rq as specified above.

Shown in FIG. 10 is a support structure 100 for an x-ray window. Thesupport structure 100 can comprise a support frame 101 defining aperimeter 104 and an aperture 105. A plurality of ribs 102 can extendacross the aperture 105 of the support frame 101 and can be carried bythe support frame 101, with openings 103 between the ribs 102. Thesupport frame 101 and the plurality of ribs 102 can comprise a supportstructure 100. The support structure 100 can comprise a carbon fibercomposite (CFC) material. The CFC material can comprise carbon fibersembedded in a matrix. Carbon fibers 12 in the composite can besubstantially aligned with a direction Al of the ribs, with at least onedirection of the ribs if the ribs extend in multiple directions, or withall directions of all ribs if the ribs extend in multiple directions.

Carbon fibers in a carbon fiber composite can be graphitic, and thus canbe highly resistant to chemical etching. Alternative methods have beenfound for etching or cutting micro-sized structures in CFC wafers in thepresent invention. The support structure 100 may be made by cutting aCFC wafer to form ribs 102 and openings 103. The CFC wafer may be cut bylaser milling or laser ablation. A high power laser can use short pulsesof laser to ablate the material to form the openings 103 by ultrafastlaser ablation. A femtosecond laser may be used. A nanosecond pulsed YAGlaser may be used. Ablating wafer material in short pulses of high powerlaser can be used in order to avoid overheating the CFC material.Alternatively, a non-pulsing laser can be used and the wafer can becooled by other methods, such as conductive or convective heat removal.The wafer can be cooled by water flow or air across the wafer. The abovementioned cooling methods can also be used with laser pulses, such as afemtosecond laser, if additional cooling is needed.

As shown in FIG. 11, the support structure 100 can have a thicknessTh_(s) of between 10-500 micrometers. Tops of the ribs 102 and supportframe 101 can terminate substantially in a single plane 116. A film 114can be disposed over, can be carried by, and can span the plurality ofribs 102 and can be disposed over and can span the openings 103. Thefilm 114 can be configured to pass radiation therethrough, such as bybeing made of a material and thickness that will allow x-ray radiationto pass through with minimal attenuation of x-rays and/or minimalcontamination of the x-ray signal.

As described above regarding FIGS. 6-9, a polyimide layer 61 can becured abutting the CFC stack 71. The polyimide layer 61 can be cut intopolyimide ribs 111 and a polyimide support frame 112, with openings 103between the ribs 111, along with the CFC stack 71. The polyimide ribs111 and the polyimide support frame 112 can be part of the supportstructure 100. The polyimide ribs 111 and the polyimide support frame112 can be disposed between the CFC stack 71 and the film 114.

A surface of the support structure 100 facing the film can have lowsurface roughness. This surface can be CFC 71 or can be polyimide 61.This surface can have a root mean square surface roughness Rq of lessthan 300 nm in one aspect, or between 30 nm and 300 nm in anotheraspect, in an area of 100 micrometers by 100 micrometers. This surfacecan have a root mean square surface roughness Rq of less than 500 nm inone aspect, or between 50 nm and 500 nm in another aspect, along a lineof 2 millimeter length. This surface can have a root mean square surfaceroughness Rq of less than 200 nanometers in one aspect, or between 20 nmand 200 nm in another aspect, in an area of 100 micrometers by 100micrometers.

The ribs 102 can have a strain at fracture of greater than 0.01 in oneaspect, greater than 0.03 in another aspect, greater than 0.05 inanother aspect, or between 0.01 and 0.080 in another aspect. Strain canbe defined as a change in length caused by a force in a directionparallel with the ribs divided by original length. If fibers aredirectionally aligned, the force F can be aligned parallel with thefibers.

The wafers described herein can also be micropatterned by laser ablationand/or water jet to form other structures, such as a flexure mechanicalmechanism, a mesoscale mechanical mechanism, a microscale mechanicalmechanism, and/or elements in a microelectromechanical system (MEMS).

As shown in FIG. 12, an x-ray window 125, including a support structure100 and a film 114, can be hermetically sealed to a housing 122. Thehousing can contain an x-ray detector 123. The x-ray detector can beconfigured to receive x-rays 124 transmitted through the window, and tooutput a signal based on x-ray energy.

What is claimed is:
 1. A method of making a wafer, the methodcomprising: a. providing a stack of at least one sheet of carbon fibercomposite (CFC) including carbon fibers embedded in a matrix, the stackhaving a first surface and a second surface; b. pressing the stackbetween first and second pressure plates with a porous breather layerdisposed between the first surface of the stack and the first pressureplate; and c. curing by heating the stack to a temperature of at least50° C., defining a first curing process.
 2. The method of claim 1,further comprising disposing a solid, polished layer between the secondsurface of the stack and the second pressure plate during the firstcuring process with the polished layer having a root mean square surfaceroughness Rq of less than 300 nm in an area of 100 micrometers by 100micrometers, on a side facing the stack.
 3. The method of claim 1,further comprising: a. releasing pressure from the stack; b. removingthe porous layer from the stack; c. disposing a polished layer on eachside of the stack, the polished layers having a root mean square surfaceroughness Rq of less than 300 nm in an area of 100 micrometers by 100micrometers, on a side facing the stack; and d. pressing the stack andpolished layers between first and second pressure plates; e. curing byheating the stack to a temperature of at least 50° C., defining a secondcuring process.
 4. The method of claim 1, wherein the porous breatherlayer comprises a porous polymer layer facing the stack and a nylon meshfacing the first pressure plate.
 5. The method of claim 1, whereinproviding the stack includes a sheet of polyimide disposed adjacent tothe second surface of the stack.
 6. The method of claim 1, whereincuring by heating the stack includes creating a vacuum of less than 50torr between the pressure plates, and maintaining the vacuum through atleast 50% of the curing process.
 7. The method of claim 1, wherein eachsheet in the stack has a thickness of between 20 to 350 micrometers. 8.The method of claim 1, further comprising micropatterning the wafer bylaser ablation, water jet, or combinations thereof to form an x-raywindow support structure comprising: a. a support frame defining aperimeter and an aperture; b. a plurality of ribs extending across theaperture of the support frame and carried by the support frame; c.openings between the plurality of ribs; and d. the support frame and theplurality of ribs comprising a support structure.
 9. The method of claim1, wherein at least 90% of the carbon fibers have a diameter of between2 and 10 micrometers.
 10. The method of claim 1, wherein the matrixcomprises a material selected from the group consisting of polyimide,bismaleimide, epoxy, or combinations thereof.
 11. The method of claim 1,wherein the matrix comprises a material selected from the groupconsisting of amorphous carbon, hydrogenated amorphous carbon,nanocrystalline carbon, microcrystalline carbon, hydrogenatednanocrystalline carbon, hydrogenated microcrystalline carbon, orcombinations thereof.
 12. The method of claim 1, wherein the matrixcomprises a ceramic material selected from the group consisting ofsilicon nitride, boron nitride, boron carbide, aluminum nitride, orcombinations thereof.
 13. The method of claim 1, further comprising apolyimide sheet cured together with and abutting the at least one sheetof carbon fiber composite.
 14. The method of claim 13, wherein thepolyimide sheet has a thickness of between 0.1-100 micrometers.
 15. Themethod of claim 1, wherein the matrix comprises polyimide.
 16. Themethod of claim 1, wherein the matrix comprises bismaleimide.
 17. Awafer formed by the method of claim 1, wherein the wafer comprises: a. awafer thickness of between 10-500 micrometers; b. at least one side ofthe wafer having a root mean square surface roughness Rq of less than300 nm in an area of 100 micrometers by 100 micrometers and less than500 nm along a line of 2 millimeter length; c. a yield strength atfracture of greater than 0.5 gigapascals (GPa), wherein yield strengthis defined as a force, in a direction parallel with a plane of a side ofthe wafer, per unit area, to cause the wafer to fracture; and d. astrain at fracture of more than 0.01, wherein strain is defined as thechange in length caused by a force in a direction parallel with a planeof the wafer divided by original length.
 18. The wafer of claim 17,wherein the yield strength is between 2 GPa and 3.6 GPa.
 19. The waferof claim 17, wherein the root mean square surface roughness is less than200 nanometers in an area of 100 micrometers by 100 micrometers.
 20. Thewafer of claim 17 micropatterned by laser ablation, water jet, orcombinations thereof to form an x-ray window support structurecomprising: a. a support frame defining a perimeter and an aperture; b.a plurality of ribs extending across the aperture of the support frameand carried by the support frame; c. openings between the plurality ofribs; and d. the support frame and the plurality of ribs comprising asupport structure.